For the pre-operative calculation of the power of an intraocular lens, three input parameters are needed: the axial length of the eye (AL), the refractive power of the cornea, and the distance between the front of the cornea and the back focal plane of the intraocular lens, the so-called effective lens position (ELP).
To a good approximation, the post-operative axial length can be substituted by the corresponding value measured pre-operatively. The axial length can be measured either ultrasonically or optically using partial coherence interferometry (PCI). Also—at least for eyes that have not undergone keratorefractive surgery—the post-op corneal power can be predicted based on the pre-op measurement of the front surface corneal radii. This prediction is based on assumptions about the corneal index of refraction and the ratio of front and back surface corneal radii. Keratometry can be measured using manual or automatic optical keratometers, or extracted from a corneal topography obtained via Placido ring projection.
The effective lens position, on the other hand, is inherently a post-operative value. In fact the final position of an IOL does not manifest itself until a number of weeks after surgery, when the capsular bag has shrunk around the implant. A pre-op parameter the ELP approximately corresponds to is the distance from the front of the cornea to the front of the crystalline lens, the so-called anterior chamber depth (ACD). The ACD can be measured ultrasonically or optically using slit projection, or it can be predicted based on the diameter of the clear cornea (the so-called white-to-white distance, WTW) and its central curvature. In commonly used IOL calculation formulas, the ELP is predicted using an empirical fit of several parameters such as ACD and AL. Olsen has suggested that the prediction can be improved by inclusion of additional parameters such as the lens thickness (LT), corneal radius, and pre-op refraction (Acta Ophthalmol. Scand. 2007: 85: 84-87). Most commonly used IOL calculation formulas are based on the same vergence formula to model focusing by the intraocular lens; they only differ in the method for predicting ELP.